In manufacturing semiconductor devices such as LSI and super-LSI or in manufacturing a liquid crystal display panel or the like, a pattern is made by irradiating a light to a semiconductor wafer or a glass plate for liquid crystal, but if a dust particle is sticking to a photomask or a reticle (hereinafter merely referred to as “photomask”) used in this stage, the particle absorbs the light or bends it, causing deformation of the pattern being transferred, edge blurring, or blackening of the base plate, and such phenomena lead to lowering of the precision of the size, grade and appearance of the end products.
Thus, these works are usually performed in a clean room, but, even in a clean room, it is yet difficult to keep the photomask clean all the time; hence, in practice, the light irradiation is conducted only after a surface of the photomask is sheltered by a pellicle as a dust fender. Under such circumstances, foreign particles do not directly adhere to the surface of the photomask, but only onto the pellicle membrane, and thus by setting a photo focus on a lithography pattern on the photomask, the foreign particles on the pellicle membrane fail to transfer their shadows onto the photomask and thus no longer become a problem to the image transfer performance.
In general, a pellicle is built up of a pellicle frame, which is an endless frame bar usually made of aluminum, a stainless steel, or the like, and a transparent pellicle membrane usually made of cellulose nitrate, cellulose acetate, a fluorine-containing polymer or the like which transmit light well; this pellicle membrane is attached via dried solution or adhesive to one of the two annular faces (hereinafter referred to as “upper annular face”) of the pellicle frame. On the other one of the two annular faces of the frame (hereinafter referred to as “lower annular face”) is laid an agglutinant layer made of a polybutene resin, a polyvinyl acetate resin, an acrylic resin, a silicone resin or the like for attaching the pellicle frame to the photomask, and over this agglutinant layer is laid a releasable liner (separator) for protecting the agglutinant layer.
A pellicle thus constructed is subjected to a foreign material inspection in a dark room with a focus lamp, so as to assure that the pellicle is used free of the effect of foreign particles. This inspection is conducted with respect to not only the pellicle membrane but also the inner wall of the pellicle frame. A foreign material stuck on the inner wall of the pellicle from is liable to fall on the mask surface so that, in general, this inspection is conducted in a darkroom using an irradiation of a light having a high illumination intensity such as one created by focus lamp, and the inspection is made by eye, and when a foreign material exists on the pellicle frame it is observed as an illuminating point. However, the surface of the pellicle frame has microscopic irregularities, which reflect light so that adding this an observer may see numerous illuminating points. On account of this it is customary that an observer compares by eye the size of an illuminating point with a certain threshold size to determine whether or not the illuminating point is a foreign material to render the pellicle unacceptable.
Now, in the case wherein the inner wall of the pellicle frame is coated with an agglutinant layer, even if a foreign material is stuck on the inner wall, the possibility that the foreign material falls is very low, so that the size of the foreign material that renders the pellicle unacceptable becomes larger such as several tens of micrometers; however, in recent years, it is becoming less common to provide agglutinant layer to the inner wall of the pellicle frame in fear that the agglutinant layer undergoes degradation and emits gas as a result of being attacked by stray lights during the light exposure operation, and thus even a very small foreign material such as the size of several micrometers would, when detected, render the pellicle unacceptable, and thus the rate of defective pellicle has been increased pellicles would increase.
However, in this stringent inspection, there are no small number of instances wherein a defective point in the alumite (anodic oxide coating) layer covering the pellicle frame is mistaken for a foreign material stuck on the pellicle frame. A pellicle frame commonly used is made of an aluminum alloy and its surface is provided with a black alumite treatment, and if a large crystalline particle (intermetallic compound) is formed in the surface, that portion of the surface fails to support a formation of a normal anodic oxide layer but tends to form a void or a white spot instead of being colored in black.
There have been trials made in order to prevent this phenomenon, and among them there are teachings according to IP Publications 1 and 2, in which the impurities in the aluminum alloy are reduced so as to minimize the size of the crystallization. However, various elements are included as supplemental ingredients in the aluminum alloy so that it is very difficult to completely do away with defects (light reflective points) that result from such alloy ingredients.
On the other hand, according to the teaching of IP Publication 3, in consideration of a fact that the surface of the alumite layer is porous and as such it tends to create dust particles, a resin coating is laid over the surface of the pellicle frame so as to suppress dust creation; however, in this method the resin coating imparts a modicum of gloss to the surface of the pellicle frame with a result that a defect (light reflective point) in the surface becomes more remarkable and invites a mistaken inspection.
Therefore, the fact remains that no pellicle frame and thus no pellicle have been realized which have such a high surface quality that there is no defect (light reflective point) that is liable to be mistaken for a foreign material. Accordingly, it has been necessary to spend a very long time to finish the inspection of the inner wall of the pellicle frame for foreign materials, and such time-consuming inspection for foreign materials has posed problems not only in cost but also in reliability of the products because of the high possibility of defective inspection in terms of mistaken identification and overlooking.